Intravascular simply means "inside a blood vessel". An intravascular ulrasound scan is one where a tiny probe is inserted inside a blood vessel to take a scan inside the body. Magnetic Resonance (MR,MRI) MR uses very strong magnetic fields to distinguish the different magnetic properties of different forms of tissue - in particular, their water content. The images that an MR scanner generates can give unrivalled levels of interior detail, as in this scan of the head